Some Thoughts on Zeroing
by Fr. FrogProperly zeroing one’s firearms is of paramount importance. As received from the factory, firearms have only a rudimentary “zero” if any, which may or may not correspond to where the bullet will actually impact when fired by a given individual. Thus it is necessary to establish a proper zero with your firearm using your particular ammunition. In addition, you have to zero your firearm yourself because each individual holds a firearm differently which can affect where the bullet strikes. While someone may be able to come close to your zero you will need to fine tune it personally.
While this page is primarily directed toward rifle users, the techniques are adaptable to handguns. I also assume in this discussion that you know proper sight alignment, trigger control, and proper shooting positions. If you don’t know these things you will have a hard time zeroing any firearm, and then even if you achieve a zero you’ll have a hard time hitting anything. This is especially true if you have poor trigger control. I have a friend who was complaining to me that his fixed sight pistol was shooting very low and to the left. I tried it and it was dead on. I then watched him shoot it and saw him not only jerking the trigger but also convulsing his grip. I worked with him to overcome these problems and the pistol was dead on for him too.
The first step in achieving a zero is to get the strike of the bullet close to where you want it. In order to do this expeditiously you will need to know the amount of movement provide by your sighting device per graduation of its dial or mechanical “click.” This information is available from your sight’s manufacturer. Keep in mind that most devices are calibrated to give a specific movement at 100 yards, and that if you shoot at a shorter range that the movement is proportionally reduced. The chart below shows what happens.
Movement per graduation or click at a given distance:
| 100 yards | 50 yards | 25 yards |
| 1 inch | .5 inch | .25 inch |
| .5 inch | .25 inch | .125 inch |
| .25 inch | .125 inch | .063 inch |
Thus if you are shooting at 25 yards with a sight graduated in 1″ marks or mechanical clicks for 100 yards and you are 2 inches off of your point of aim, at 25 yards you will need to move your sight’s adjustment eight graduations or clicks.
For fixed sights, the problem is a little different since you need to either physically move the rear sight for deflection, or alter the height of either the front or rear sight for elevation. The formula to determine the amount of adjustment is:
M = S * D / (R * 12)
where
M = amount of movement or change in sight height needed (in inches)
D = distance (in inches) needed to move the bullet’s strike to hit point of aim
R = range to target (in feet)
S = distance between the front edge of the rear sight and the rear edge of the front sight blade (in inches)
Note that to move a bullets strike horizontally, move the rear sight in the direction you need to go. To adjust elevation, either lower the front sight or raise the rear sight to raise the bullets impact, or raise the front sight or lower the rear sight to lower the bullet’s impact. As an example, if your sight radius is 5″ and you need to move the bullet’s strike 3″ higher at 25 yards (75 feet):
M = 5 * 3 / (75 *12) = .016
thus you need to lower the front sight (or raise the rear sight) by .016 inches.
Getting on the (center of the) Paper
Now that you know how to make the adjustments you need to first get the bullet’s strike close to where you want it and to then refine your zero. It helps if you have a good ballistics program like ShootingLabTM to compute trajectory data so you know where you should be at a given range.
Most folks will do a zero (or at least an initial zero) at 25 yards and the go from there. The military does this. However, if you stop there you can still be way off at longer ranges. The procedures below are much better.
You can do this in a couple of ways, here are two:
The Quickie
- Using your best shooting technique fire one shot at a distinct aiming point at 100 yards from the bench.
- Sandbag your rifle so that the crosshairs are on your original aiming point.
- Without moving your rifle, adjust the crosshairs so they are centered on the bullet hole.
- Fire one round at the aiming point. It should be dead on. You can then make a compensating elevation adjustment to get your long range zero.
The Old Fashion Way
You will need access to a range with at least 100 yards and preferably 200 yards, a shooting bench, and you will need to have someone with you as a spotter with a spotting scope or binoculars. You should start with your bore clean and dry and all sight mounting and stock screws tight. Done at shorter ranges this procedure is suitable for handguns and shotguns.
- Pick out a distinct aiming point (not a paper target) on your backstop, preferably, 200 yards. Holding dead center on the aiming point very carefully fire 1 round. If your spotter tells you that your shot was dead on or within an inch or two skip to step 4.
- Adjust your sights in the horizontal plane, using a bold correction, and fire one shot. Don’t creep up on the aim point, use bold adjustments. Once you have a “left” and “right” you can split the difference and be on target.
- Adjust your sights in the vertical plane, using bold corrections, and fire one shot. Don’t creep up on the aim point, again, use bold adjustments. Once you have a “over” and an “under” you can split the difference and be on target. If steps 2 and 3 are done right you will be on target with a maximum of 4 shots.
- Using a paper target with a distinct aiming point at 100 yards, carefully fire two shots using your best bench technique. They should be very close to your aiming point. If your initial firing was at 200 yards, the center of your two shots will be about 2.75″ to 3″ high at 100 yards. If your initial firing was at 100 yards you will be close to dead center. Make appropriate corrections to put your point of aim 2.75″ to 3″, or the distance you prefer, directly above your point of aim. Do not not make minor corrections to your scope. If you need to move the point of impact say, 4 inches, move 6 inches and then back 2 inches. Tap the side of your scope after making the adjustment to insure the reticle is centered and settled. Fire 2 more shots and repeat as necessary. Unless you do not not have a 200 yard range, this step doesn’t have to be perfect, just very close.
- At 200 yards fire 3 shots as carefully as you can at a paper target with a distinct aiming point. The center of your group should be centered about or just slightly (about a inch and a half) above your aiming point. If not, make the appropriate corrections using a 3 shot group. At this step you want to be very precise.
- Abandon the shooting bench. Set up a graphic target (animal or tactical) at 200 yards, or as close as you can get to that range, and from a field shooting position (sitting or prone work best) carefully fire a 5 shot group. Your 5 shots should be evenly grouped about your point of aim. If the group is larger than 10″ fatigue has probably set in (or you are not the marksman you thought you are). Rest for 30 minutes, think about trigger control and proper position, and try again.
Using the procedure in steps 1 through 5, you can get a bench zero on an unknown rifle with a little as 6 rounds if you are careful, and maybe a maximum of 10 rounds. If you are simply changing your load with a previously zeroed rifle you can probably start with step 5.
If you don’t have anyone to observe for you it may be necessary to fire at a paper target with a distinct aim point and a grid (possibly at a closer range - say 25 yards) and adjust the sights to center the shot.
Handguns should be fired either from a sand bag rest or from the braced sitting position, and shot initially at 50 yards (or 100 yards if you are really good). Shotguns with slugs should start at 75 yards.
Keep in mind that with all of the above techniques you have to shoot very carefully, without jerking the trigger or disturbing the sights. If you jerk the trigger you’ll never be able to obtain a good zero.
The Rifle
Please note velocity and drop figures have not been rounded to eliminate meaningless precision as on the External Ballistic page. Keep in mind that knowing velocity within 10 f/s and drop within 1/10th of an inch under 100 yards, and within 1 inch past that range, is precise enough for all but techno-weenies.
Too many shooters zero their rifles (for that matter, all their firearms) at too short a distance and thus lose the advantages of a more useful trajectory. For the most efficient use of trajectory, you want to zero your firearm for the farthest distance over which the actual point of impact vs. point of aim will be within the critical (vital) zone of your expected target, from the muzzle to as far away as possible. That is, the range at which the maximum ordinate or maximum height of the bullet’s trajectory and the drop will not be greater than one half of the intended target’s vital zone.
If you zero for too close a range, you make hitting at greater distances much more difficult. While you may say, “but I’ll never shoot anything at a greater distance” one never knows when that trophy buck or a target of opportunity may show up way over there. With a proper zero, one doesn’t have to worry about hold over (or under) until the ranges approach 300 yards, so why not take advantage of what you have, even if all you do is hunt in the thick brush of New England.
Your critical zone size will vary depending on your intended target but ± 3 - 4 inches is a good compromise for most uses. The table below gives the diameter of the vital zone and suggested maximum ordinate for some common game.
| Vital Zone Diameter |
Suggested Maximum Ordinate |
|
| Varmints and small game (squirrels, woodchucks, etc…) |
3″ - 5″ | 1.5′ |
| Light game (small deer, wild boar, etc…) |
6″ - 8″ | 3″ |
| Medium game (white tail deer, bear, etc…) |
10″ | 3″ - 4″ |
| Large game (Moose, elk,etc.) | 15″ | 6″ |
The table below, based upon the 7.62 mm NATO GI M80 ball round (G7 = .195) shows the effect of different zeroing ranges. The effect of differing zero ranges will still hold true, for other bullet weights and types. You can see that a good zeroing range for the .308 / 150gr is somewhere between 200 and 250 yards depending on the individual projectile, if we accept a vital zone diameter of between 6 and 8 inches. Interestingly this also works out very closely for other bullet weights and cartridges with a muzzle velocity in the range of 2400 to 2900 f/s. If we zero our rifles to be between 2.75 and 3.25 inches high at 100 yards we will pretty much be in the ball park no matter what bullet we zero with.
| Range | Vel. | 100 yd Zero | 200 yd Zero | 225 yd Zero | 250 yd Zero | 300 yd Zero |
| 0 | 2750 | -1.50 | -1.50 | -1.50 | -1.50 | -1.50 |
| 50 | 2634 | -0.12 | 0.87 | 1.20 | 1.55 | 2.30 |
| 100 | 2520 | 0 | 1.99 | 2.65 | 3.35 | 4.85 |
| 150 | 2410 | -1.25 | 1.75 | 2.73 | 3.78 | 6.03 |
| 200 | 2302 | -3.99 | 0 | 1.31 | 2.71 | 5.72 |
| 250 | 2197 | -8.37 | -3.39 | -1.74 | 0 | 3.76 |
| 300 | 2094 | -14.56 | -8.58 | -6.60 | -4.51 | 0 |
| 350 | 1995 | -22.73 | -15.75 | -13.45 | -11.00 | -5.74 |
| 400 | 1898 | -33.08 | -25.10 | -22.47 | -19.68 | -13.67 |
| 450 | 1803 | -45.85 | -36.87 | -33.92 | -30.78 | -24.01 |
| 500 | 1710 | -61.29 | -51.32 | -48.04 | -44.55 | -37.03 |
| 550 | 1619 | -79.72 | -68.75 | -65.13 | -61.30 | -53.03 |
| 600 | 1530 | -101.46 | -89.49 | -85.55 | -81.36 | -72.34 |
| MO = 0.1″ @ 81.6 yd VZ = .2″ PBR = 108.5yd |
MO = 2.1″ @ 117.2 yd VZ = 4.2″ PBR = 233.4yd |
MO = 2.9″ @ 130.4 yd VZ = 5.8″ PBR = 263.5yd |
MO = 3.8″ @ 141.7 yd VZ = 7.6″ PBR = 293.5yd |
MO = 6.1″ @ 166.7 yd VZ = 12.2″ PBR = 352.7yd |
||
MO = Maximum Ordinate
VZ = Vital Zone
PBR = Point Blank Range
In the interest of completeness lets look at the effect of ballistic coefficient on optimum zero. In the table below I have taken the worst, average, and best case published ballistic coefficients for standard commercial .30 caliber 150gr pointed (flat base and boat tail) bullets at a muzzle velocity of 2750 f/s and assumed a maximum ordinate of 3 inches.
Effect of BC on Optimum Zero Range and
PB Range Commercial .30 cal 150gr Pointed Bullets
MV = 2750 f/s
3″ Maximum Ordinate
| Range |
Path - Worst Case (G1 = .301) |
Path - Average (G1 = .383) |
Path - Best Case (G1 = . 435) |
| 0 | -1.50 | -1.50 | -1.50 |
| 50 | 1.28 | 1.26 | 1.24 |
| 100 | 2.78 | 2.76 | 2.74 |
| 150 | 2.84 | 2.88 | 2.89 |
| 200 | 1.27 | 1.51 | 1.59 |
| 250 | -2.14 | -1.52 | -1.29 |
| 300 | -7.62 | -6.36 | -5.88 |
| 350 | -15.45 | -13.21 | -12.34 |
| 400 | -25.98 | -22.27 | -20.83 |
| 450 | -39.58 | -33.77 | -31.55 |
| 500 | -56.69 | -47.99 | -44.69 |
| 550 | -77.82 | -65.22 | -60.49 |
| 600 | -103.55 | -85.79 | -79.20 |
| Zero = 222.2 yd PB = 259.4 yd MO = 3″ @ 127.4 yd |
Zero = 228.4 yd PB = 267.6 yd MO = 3″ @ 131.5 yd |
Zero = 231.1 yd PB = 271.1 yd MO = 3″ @ 132 yd |
Notice that if one zeros to be about 2.75 inches or a little higher with any 150gr pointed bullet, that you will pretty much have an optimum zero assuming a 6 inch vital zone and have a point blank range of 265 yards ±. We could pick nits but for practical field use you’ll be just fine. If we choose a different maximum ordinate things still hold up nicely.
If we are using a cartridge with a velocity substantially higher than the above, we can use it to get a a longer point blank range by using the same 100 yard point of impact as above. If you want to refine things, this is a great reason to acquire a good ballistics program and by playing with different parameters you can learn a lot. I highly recommend RSI’s Shooting Lab.
To lazy to work out the details? Then zero rifles as follows, using the longer zero for the higher velocity range. This will at least get you close to a decent working zero for typical sporting firearms with sight heights of about 1.5″.
| Muzzle Velocity (f/s) | Suggested Zero Range (yards) |
| 1800 - 2400 | 175 - 225 |
| 2400 - 2900 | 225 - 250 |
| 2900 and higher | 250 - 300 |
Disclaimer - As far as I know all the information presented above is correct and I have attempted to insure that it is. However, I am not responsible for any errors, omissions, or damages resulting from the use or misuse of this information, nor for you doing something stupid with it.
Updated 2008-06-11
from Fr. Frog’s Pad